Anyone know

Where can I get a moderately priced CO2 controller and CO2 PPM reader? By moderately I mean not one of these 500.00 jobs.

Try craigslist. And a cheep co2 ppm reader is the seringe type sold at hydrostores. But if you want digi readers don't cheep out. Ryle of thumb you get what you pay for.

OK you want cheap? try this
http://www.co2meter.com/collections/co2-sensors/products/k-22-oc-co2-sensor-module
$100. it has a built in output that will trigger your regulator when CO2 drops below 800 ppm and off at 1000 ppm. not ideal but if you place it up high where the CO2 levels are lowest......
a $40 cable will allow you to log CO2 and may allow you to set on/off levels & they make another unit that will log temps & humidity as well.